Because of the high package density attainable with multilayer ceramic (MLC) substrate circuit structure, it has achieved extensive acceptance in the electronics industry for the packaging of semiconductor integrated circuit semiconductor devices, and other elements; as for example, see U.S. Pat. No. 3,379,943 granted Apr. 23, 1968 to J. D. Breedloff, U.S. Pat. No. 3,502,520 granted Mar. 24, 1970 to B. Schwartz, U.S. Pat. No. 4,080,414 granted Mar. 21, 1978 to L. C. Anderson, et al, and U.S. Pat. No. 4,234,367 to L. W. Herron, et al granted Nov. 18, 1980.
In general, these ceramic structures are formed from ceramic green sheets, which sheets are prepared from a ceramic slurry by mixing a ceramic particulate, a thermoplastic polymer (e.g. polyvinylbutyral) and a solvent. This mixture is then cast or spread onto ceramic sheets or slips from which the solvents are subsequently volatilized to provide a coherent and self-supporting flexible green sheet. These green ceramic sheets are laminated to form a substrate which is fired to drive off and/or burn off the resin, and subsequently sintered to fuse the ceramic particulates together into a densified ceramic substrate.
In the fabrication of multilayer ceramic structures, an electrically conductive paste composition is deposited in patterns on pre-punched green sheets, which when laminated and sintered collectively form the desired internal metallurgy circuit structure. The component green ceramic sheets have via or feed through holes punched in them as required for level interconnection in the ultimate structure. The required number of component green sheets are stacked on each other in the required order. The stack of green sheets is then compressed or compacted at the necessary temperatures and pressures to affect a bond between adjacent layers. The overall process is more completely described in "Ceramics For Packaging" by D. L. Wilcox, Solid State Technology, Feb. 1971 P. 55-60, and in an article entitled "A Fabrication Technique For Multilayer Ceramic Modules" by H. D. Kaiser et al, Solid State Technology, May 1972 P. 35-40.
Alumina, Al.sub.2 O.sub.3, because of its excellent insulating properties, thermal conductivity, stability and strength has received wide acceptance as the material of choice for fabrication of such MLC substrates. However, for various high performance applications, the relatively high dielectric constant of alumina causes significant signal propagation delays and noise. A further disadvantage of alumina is its relatively high thermal expansion coefficient difference, on the order of 65 to 70.times.10.sup.-7 per .degree. C., as compared to monocrystalline silicon with a coefficient from 25 to 30.times.10.sup.-7 per .degree. C. This difference may in certain cases result in some design and reliability concerns, particularly when a silicon device chip is joined to the substrate with solder interconnections.
A particular disadvantage is that the high sintering and maturing temperature of commercial alumina (about 1600.degree. C.), restricts the choice of co-sinterable conductive metals to refractory metals, such as tungsten, molybdenum, platinum, palladium, or a combination thereof. The high sintering temperature precludes the use of metals with higher electrical conductivities such as gold, silver or copper because the latter would be molten before the sintering temperature of alumina is reached.
A multilayer glass ceramic structure is disclosed and claimed in U.S. Pat. No. 4,341,324 by A. H. Kumar et al, whose teachings are incorporated herein by reference thereto, which avoids the use of and the attendant disadvantages of alumina ceramic. The disclosed multilayer glass-ceramic structures are characterized with low dielectric constants and are compatible with thick film circuitry of gold, silver, or copper and are co-fired therewith. Of the two types of glass-ceramics disclosed in the aforementioned patent, one has .beta.-Spodumene, Li.sub.2 O.sup.. Al.sub.2 O.sub.3.sup.. 4SiO.sub.2, as the principal crystalline phase while the other has cordierite, 2MgO.sup.. 2Al.sub.2 O.sub.3.sup.. 5SiO.sub.2, as the main crystalline phase. A common feature of these sintered glass-ceramics, among others, is their excellent sinterability and crystallization capability below 1,000.degree. C.
However, it was found that silver has a tendency to cause electromigration problems and is suspected of diffusing into the glass ceramic.
Although successful glass-ceramic substrates have been made using gold metallurgy with a resistivity about 3.75 microhm-centimeter, gold is however extremely expensive. This limits the choice to copper as a practical economic alternative. Also, any alloying of other less expensive metals with gold would result in the disadvantage of an increase in resistivity.
The use of copper is relatively new in the thick film technology. Because of copper's oxidizing potential, it is necessary to sinter multilayer ceramic substrates in reducing or neutral ambients. However, since reducing ambients can occasion adhesion problems, neutral ambients are preferable.
In the fabrication of multilevel glass-ceramic structures, difficulty has been encountered in removing the resin binders that are used to produce the slurry for processing.
It is generally recognized that burn-out of organic binders used for ceramic package fabrication is a very demanding process, especially in the fabrication of a glass-ceramic and Cu metallurgy system. The reducing atmosphere or low oxygen atmosphere required to prevent oxidation of Cu, typically H.sub.2 +N.sub.2 +H.sub.2 O or H.sub.2 +H.sub.2 O employed in the presintering and densification phase, leads to incomplete removal of carbon formed as a consequence of binder pyrolysis. The carbonaceous residues thus generated become an integral part of the ceramic substrate after sintering and densification.
With the use of glass-ceramics and copper metallurgy, the maximum temperature for binder removal, due to the coalescence of glass particles, is in the range of about 800.degree.-875.degree. C. Thus, after the glass has coalesced, any remaining binder residue will become entrapped in the glass body. It has also been found that nitrogen or other neutral or reducing ambients make difficult the removal of binder below the temperature of the glass coalescence, e.g. about 800.degree.-875.degree. C., which can result in black or darkened substrates that are not fully sintered. The black or darkened color is generally attributed to carbon residue trapped in the ceramic. The carbon residue can have an adverse affect on the electrical characteristics of the resultant substrate, as by reducing the resistivity and having an adverse affect on the dielectric constant of the material.
Various efforts have been made to alleviate complete binder removal. Some difficulties are encountered with various neutral or reducing atmospheres, which include wet and dry nitrogen, wet and dry forming gas, long holds at temperatures below the coalescence temperature of glass-ceramic so as not to entrap volatile products, and alternating air and forming gas for purposes of oxidizing carbon and reducing any form of copper-oxide to copper without drastic volume changes resulting from the copper oxide formation. In addition, attempts have been made to develop binder vehicle systems which would by some mechanism burn off without any remaining carbonaceous residue and thereby not have darkened the substrates. In general, these efforts resulted in other attendant disadvantages even though characterized with improved burn-out properties.
U.S. Pat. No. 4,234,367 by Herron et al, issued Nov. 18, 1980, discloses and claims a method of forming a glass-ceramic composite structure with a copper-based metallurgy. In this process, laminated green ceramic substrates are heated in a H.sub.2 /H.sub.2 O environment to a burn-out temperature in the range between the anneal and softening point of the glass-ceramic material which temperature is less than the melting point of the copper. The conditions are sufficient to remove the binder without oxidizing the copper metallurgy. The resultant binder-free laminate is then heated in a nitrogen atmosphere at a somewhat higher temperature to coalesce the glass particles. The copper is prevented from oxidizing in this later heating phase by the inert atmosphere.
While the process is operable and effective, the binder removal heating step is quite lengthy, carbonaceous residues may in some instances remain in the substrate, and the ceramic portions surrounding the copper metallurgy may be porous.